Preliminary design of the I2S-LWR containment system

Abstract

The Integral Inherently Safe Light Water Reactor (I2S-LWR) is a novel reactor design concept which aims at delivering an electric power output level comparable to that of large LWRs (approximately 1000 MWe), while at the same time achieving an overall level of safety that is enhanced with respect to large Generation III+ LWRs. One of the main safety goals is to achieve indefinite cooling following design basis accidents (DBAs) using atmosphere air as ultimate heat sink. In order to accommodate these goals, the I2S-LWR incorporates several innovative safety features, including an integral Reactor Pressure Vessel (RPV), enhanced passive Decay Heat Removal (DHR) systems and several containment passive cooling systems. The present work is focused on a passive and reliable containment design, which plays a significant role in LOCA scenario and is the last boundary to prevent the release of radioactivity to the environment. In this paper, several innovative passive systems located in the I2S-LWR containment are proposed, including Core Make-up Tank (CMT), Accumulator (ACC), Passive Suppression System (PSS), Automatic Depressurization System (ADS), Passive Containment Cooling System (PCCS) and Passive Reactor Cavity Cooling System (PRCCS). The best-estimate thermal-hydraulic code RELAP5 has been used to model the I2S-LWR RPV and containment passive systems. The inadvertent opening of PORV (Power Operated Relief Valve) accident scenario has been simulated in order to study the containment response, including the coupling between RPV and containment. The results show that, through the activation of the ADS, pressure equilibrium between containment and reactor pressure vessel is achieved, while maintaining the reactor core covered at all times. The containment passive cooling systems ensure that the containment pressure remains at acceptable levels throughout the transient. A sensitivity study on the number of operating ADS valves, and on the availability of accumulators is also performed.